T-Rex : Level 3, in the right face of the hill with all the tunnels through it.
Giga : Level 2, on a vine at the bottom of the big cliff.
Magno : Level 6, inside the top-right of the first volcano.
Iguano: Level 4, inside the hill with the tunnels, above the waterfall.
Alien : Level 5, halfway through the level inside the big meteorite on the ground.
Spino : Level 2, at the bottom of the first big cliff all the way on the left.
Oloro : Level 4, at the end of the underground part, on top of some platforms.
Rinch : Level 3 Underground on a platform, as well as in the Planet D Speedrun.
Diplo : Apocalypse, inside the top left of the last volcano.
Bronto: Inside last cave under the Insane difficulty.

Crossbones trophy:

The skull and crossbones trophy is a 10+ second "doom surf," which is when you are right on the edge of the cloud of doom. My longest doom surf was 15.5 seconds, which nearly gave me a heart attack, thank you very much.

Planet D:

You need to beat the game on Insane, and it unlocks. Tip: it unlocks even if you are using all the cheats! I'm not good enough yet to beat insane.

Rinch Egg:

It's in the Planet D level! I'm using the DNA from the Rinch egg to build up my stats to try and make it to the Bronto egg.

Giga Egg:

The Giga Egg is on Rainin' Lizards - but only on the Easy setting - won't appear on Medium and I haven't seen it elsewhere. It's at the bottom of a huge cliff about half way through.